Any new carburetion system for an internal combustion engine to be useful at all, must perform at least as well as known devices in the areas of fuel economy and exhaust emission. As will become evident, both economy and exhaust emissions are heavily dependent upon predetermined air-to-fuel ratios.
It is generally accepted in the field that reduction of harmful emissions could be accomplished by delivering a homogeneous mixture of air and fuel to the engine thereby allowing lean mixtures to be burned with complete combustion. Known state-of-the-art carburetion-induction devices utilized with the conventional internal combustion engine are capable of providing complete combustion with air-to-fuel ratios of 18.5:1. Air to fuel ratios in this range are effective in reducing hydrocarbons and carbon monoxide but do little to effectively reduce oxides of nitrogen.
If the air-to-fuel ratio were further increased to reduce oxides of nitrogen the lean limit is reached. This limit occurs when the air-to-fuel mixture in a cylinder can no longer support complete combustion. The result is a sharp increase in the emission of hydrocarbons. Although it is little known, it has been possible for applicants to achieve air-to-fuel ratios of 21:1 using a homogeneous mixture of dry fuel vapor and air without reaching the lean limit. This high air-to-fuel ratio results in drastic reductions of oxides of nitrogen while maintaining low hydrocarbon and carbon monoxide levels.
It should be recognized that the above discussion applies at relatively high power settings for the engine. For example, an RPM greater than 60% of maximum torque RPM and power at a given RPM greater than 30% of maximum torque RPM. At lower power and RPM, the lean limit decreases reaching a minimum of about 16:1 air-to-fuel ratio at idle and increasing with increasing power and RPM to the 21:1 discussed above at highway level road loads.
In view of the foregoing, it is not only necessary for a system to be susceptible to tight control of the air-to-fuel ratio, but it must also have the flexibility to allow a change in the air-to-fuel ratio which changes with changing load conditions. If the system does not have this flexibility but does have the ability to maintain a constant air-to-fuel ratio, it would only really be effective over a desired or given operating range and would not function well outside such range.
Theoretically, the higher the air-to-fuel ratio, the more thermally efficient an engine will perform. This theory is generally true for the higher power levels discussed above. Under lighter loads and speeds such as idle and low speed cruise, thermal losses and low compression levels of the fuel change resulting in reduced efficiencies with increasing air-to-fuel ratio. The preferred air-to-fuel ratios for reduced emissions are very near the preferred levels for best economy and efficiency. Thus, it is apparent that the air-to-fuel ratio must be controlled as a function of load and speed for optimum economy and efficiency.
The foregoing provision of lean air-to-fuel ratios in combination with a homogeneous mixture of dry fuel vapor and air reduce flame propagation speeds and allows the use of high compression and minimum spark together with the best torque spark advance which results in a substantially improved fuel consumption coupled with much improved emissions.
In providing a proper system for controlling the air-to-fuel ratio, it is necessary that one know the exact percentage of fuel in the vaporized fuel mixed with the incoming air. The problem involved is not in being able to vaporize a sufficient quantity of fuel, but rather that of controlling the vaporization such that the amount of fuel vaporized can be determined and thus held constant. The achievement of a dew point of a vapor and a diluent is an excellent method of attaining such a control. If a mixture of fuel vapor and diluent is at dew point and the temperature of the mixture is known, the percentage of a given vapor to diluent is known.
There have been many means devised and patented for vaporizing fuel, but none is known to control vaporization by assuring the achievement of a dew point prior to our invention as set forth in the heretofore referred to copending patent application. Without such a control, except where the fuel is predetermined prior to vaporization, it is next to impossible to precisely control the air-to-fuel ratio so essential to low exhaust pollutents and improved economy.